This invention relates to an electric power steering apparatus and, more particularly, to an electric power steering apparatus for supplying a steering assist torque from an electric motor to a steering mechanism through a torque limiter.
The electric power steering apparatus is usually provided on the electric motor shaft side with a torque limiter for absorbing excessively large impact torque acting when an excessive torque is generated on the electric motor for generating a steering assisting torque on the steering mechanism or other steering assisting torque transmission mechanism.
FIG. 5 illustrates a torque limiter of a conventional electric power steering apparatus having a similar structure to that disclosed in Japanese Patent Laid-Open No. 9-84300 and FIG. 6 is an exploded view of the torque limiter shown in FIG. 5. In these figures, the torque limiter 2 comprises a case 4 fixed to an output shaft 3 of the electric motor (not shown), a cover 5 secured onto the case 4 by caulking, a boss 6 connected to a driven shaft (not shown) and rotatable relative to the case 4 and a friction plate 8 disposed between the cover 5 and the boss 6 through a spring member 7 and rotatable relative to the boss 6. The friction plate 8 is rotatable relative to the boss 6 but is arranged to rotate together with the case 4 by the interlocking rotation limiting portion (not shown) formed on the case 4. Also, the friction plate 8 is urged against the boss 6 by the action of the spring member 7. The cover 5 supports the urging force of the spring member 7 and is secured to the case 4 by the caulking of the inner circumferential edge portion.
During the ordinary operation, the steering assist torque from the output shaft 3 of the electric motor is transmitted to the driven shaft (not shown) through the case 4 and the boss 6 integral to the case 4 because of the frictional force generated between the friction plate 8 and the boss 6 and between the boss 6 and the case 4 due to the pressure from the spring member 7. When an excessive impact force acts against the electric motor side from the wheel side through the driven shaft, a slip generates between the friction plate 8 and the boss 6 and between the boss 6 and the case 4 of the torque limiter 2, permitting the boss 6 to rotate relative to the case 4 and the output shaft 3 to absorb the impact torque.
FIGS. 7 to 9 illustrate another conventional torque limiter 10. This torque limiter 10 comprises a case 11 fixed to an output shaft 3 of the electric motor (not shown), a boss 12 spline-engageable to the driven shaft (not shown) and concentrically rotatable relative to the case 11, and a ring member 13 disposed between an outer circumferential surface of the boss 12 and an inner wall surface of the case 11. The ring member 13 has a trapezoidal cross-sectional shape for example and comprises a projection portions 14 that abut to the inner wall surfaces of the inner wall surface 17 of the case 11 and leg portions 15 engaged with the groove 16 formed to extend in the circumferential direction of the boss 12.
During the ordinary operation, the steering assist torque from the output shaft 3 of the electric motor is transmitted to the boss 12 and to the driven shaft (not shown) through the case 11, the case inner wall surface 17 and through the ring member 13. When an excessive impact force acts against the electric motor side from the wheel side through the driven shaft, a slip generates between the boss 12 and the case 11 of the torque limiter 10, permitting the boss 12 to rotate relative to the case 4 and the output shaft 3 integral with the case 4 to absorb the impact torque, whereby the torque transmission mechanism such as plastic gears may be prevented from being destroyed.
As discussed above, the pressure member and the friction plate are employed in the torque limiter of the electric power steering apparatus shown in FIGS. 5 and 6, so that, when some kind of damages are generated in the torque transmitting portion, the friction factor of the surface of the friction member varies to significantly change the transmitted torque and degrade the initial desired performance, also resulting in disadvantages in the maintenance.
In the torque limiter of the electric power steering apparatus shown in FIGS. 7 and 8, in addition to the transmitted torque being significantly changed as discussed above, the function of protecting the resin gear at an elevated temperature can be easily degraded because the transmitted torque has no temperature coefficient because of its nature. This point will be described later in conjunction with FIG. 3. Although Japanese Utility Model Laid-Open No. 7-35248 discloses the use of the torque limiter composed of the pressure member and the friction plate disposed on the motor output shaft as an example of the torque limiter for generating a slip upon an excessive torque, this torque limiter is disadvantageous in that it is less stable in the transmitted torque because of a wide range of the friction coefficient.
Thus, in the conventional torque limiter of the electric power steering apparatus, the transmitted torque changes according to the lapse of time during the use and the stability is insufficient and, when the temperature changes, the transmitted torque does not decrease to the predetermined value particularly at an elevated temperature, resulting in a less sufficient reliability and in a failure to provide the maintenance-free feature. Also, while the torque limiter itself already has a simple structure, an electric power steering apparatus having an inexpensive torque limiter that has further improved durability and cost due to the present demands in the market is desired.
Accordingly, an object of the present invention is to provide an electric power steering apparatus having a torque limiter free from the above-discussed problems of the conventional torque limiter.
Another object of the present invention is to provide an electric power steering apparatus having an inexpensive torque limiter that can cope with the temperature change with a simple structure.
With the above object in view, the present invention resides in an electric power steering apparatus for supplying a steering assist torque from an electric motor to a steering mechanism through a torque limiter, the torque limiter comprising a case made of a magnetic material and secured to an output shaft of the electric motor, a boss made of a magnetic material concentrically rotatably arranged relative to the case and connected to a driven shaft, and a permanent magnet member disposed between the case and the boss for magnetically connecting therebetween.
The permanent magnet member may be an annular member having an axially anisotropic magnetization and having a negative temperature coefficient of residual magnetic flux density.
The case and the boss may be ferromagnetic magnetic material.
The electric power steering apparatus may further comprise a layer of an abrasion-resistant material provided between the permanent magnet member and at least one of the case and the boss.
At least one of the case and the boss may comprise a guide portion for guiding and supporting at least one of an outer circumference surface and an inner circumference surface of the permanent magnet member that is annular.
At least one of the case and the boss may comprise a seal portion for sealing at least one of the outer circumferential surface and the inner circumferential surface of the other of the case and the boss against the entry of any foreign matters.
The electric power steering apparatus may further comprise a cover for covering the outside of the case and the boss.
The guide portion of at least one the case and the boss may constitute a portion of a magnetic circuit for permitting the magnetic flux to pass therethrough.